1. Field of the Invention
The present invention relates to chiral diphosphorus compounds and their transition metal complexes, to a process for preparing chiral diphosphorus compounds and their transition metal complexes and also to their use in asymmetric syntheses.
2. Brief Description of the Prior Art
Enantiomerically enriched chiral compounds are valuable starting substances for preparing agrochemicals and pharmaceuticals. Asymmetric catalysis for the synthesis of such enantiomerically enriched chiral compounds has gained great industrial significance.
The multitude of publications in the field of asymmetric synthesis shows clearly that transition metal complexes of diphosphorus compounds are particularly suitable as catalysts in asymmetric reactions. In particular, transition metal complexes of diphosphorus compounds have found use in industrial processes as catalysts in asymmetric hydrogenations of C═O, C═N and C═C bonds, hydrocyanations and hydroformylations.
For instance, U.S. Pat. No. 5,175,335; Rajan Babu, J. Am. Chem. Soc., 1996, 118, 6325–6326 and Rajan Babu, J. Org. Chem., 1997, 62, 6012–6028 disclose the use of enantiomerically enriched 1,6-substituted 3,4-(bisphosphinito)tetrahydrofurans and their transition metal complexes for asymmetric hydrocyanations and hydrogenations.
The use of the enantiomerically enriched 3,4-(bisphosphino) tetrahydrofurans and their transition metal complexes in asymmetric hydrogenations is also disclosed by EP-A 885 897 and A. Terfort, Synthesis, 1992, 10, 951–953. Enantiomerically enriched 3,4-(bisphosphito)tetrahydrofurans are described, for example, in W. R. Jackson, Aust. J. Chem., 1982, 35, 2069–2075 and, 3,4-(phosphinophosphito)tetrahydrofurans in A. Kless, Tetrahedron: Asymmetry, 1996, 7, 33–36.
The disadvantage of all of the enantiomerically enriched 3,4-(diphosphorus)-tetrahydrofurans mentioned is that steric and electronic variation of the central tetrahydrofuran framework, which is necessary for precise optimization and adaptation of the ligand and therefore of the catalyst for a given substrate, is only possible to a very limited extent and by numerous, complex synthetic steps. These disadvantages make industrial utilisation of such ligands and the catalysts preparable therefrom uneconomic.
There is therefore a need to develop a ligand system whose steric and electronic properties can be easily varied, and whose transition metal complexes as catalysts in asymmetric synthesis, in particular asymmetric hydrogenations, enable not only high enantioselectivity but also high conversion rates.